1. Field of the Invention
The present invention relates to a device and method for determining a key used to clip a subject moving against a colored background in such a way as to be able to overlay the subject onto a new colored background. Thus, the device and method relate to any electronic device that incorporates the inventive device and method such as, for example, video mixers or autonomous devices for clipping or overlaying video images commonly referred to as "chroma-keyers". Such electronic devices or apparatuses are used, for example, in television studio equipment.
2. Discussion of the Background
Conventional systems use a clipping key to demarcate a subject video (i.e., a source image having a subject moving over a colored background) that is to be clipped from a source image as closely as possible so as to preserve the greatest detail of the subject. Examples of such details include individual hairs on a person's head, transparency of a person's glasses, or smoke from a cigarette.
The clipping key is used to distinguish between the colored background of the subject video and the subject itself. Employing the clipping key first requires that a color-luminance space (i.e., a color space) be defined so that a volume within the color space can be defined as containing the colored background. Once the volume is defined, the subject may be extracted from the colored background in order to overlay the subject against a new background.
FIG. 1 represents a conventional approach for defining a volume of the colored background in the color space. In FIG. 1, each pixel M(n) of index n of the colored background is represented by luminance component, yn, a red color difference chrominance component, crn, and a blue color difference chrominance component, cbn, of the pixel M(n). For convenience, in what follows, the color difference chrominance components will be referred to as chrominance components. Mean values y0, cb0, cr0 of each component of the colored background may then be written as follows: ##EQU1## where N is the number of pixels of the colored background corresponding to an acquisition window pointing at all or part of the colored background.
In the plane (CB, CR), the calculation of euclidian distance .rho..sub.n of a pixel of index n from the point cb0, cr0 is given by the relation: ##EQU2##
Two thresholding operations, one in the plane (CB, CR) and the other along the axis Y then make it possible to define the region of discrimination between background and subject. Thus, any point whose luminance value is between the extreme values ymin and ymax and whose euclidian distance is less than a value .rho..sub.1 belongs to the colored background 1.
Similarly, any point whose luminance value is between the values ymin and ymax and whose euclidian distance .rho..sub.n is greater than a value .rho..sub.2, itself greater than .rho..sub.1, belongs to the subject 3. Finally, any point whose luminance value is between the values ymin and ymax and whose euclidain distance .rho..sub.n is between .rho..sub.1 and .rho..sub.2 belongs to an intermediate transition region 2.
Adjustment of the contents of the image proves to be difficult with such a volume approach and the cylindrical shape of the volume described in FIG. 1 has numerous drawbacks. For example, it is difficult to process overly strong variations in the luminance of the colored background without affecting the luminance of the subject. Similarly, pixels repre senting a shadow cast by the subject cannot be incorporated into the volume of the colored background. It follows that any processing of the shadow proves to be impossible.